Nitrogen Rejection from Natural Gas
Natural gas is a mixture of methane and light hydrocarbons (ethane, propane, butanes, pentanes, and small amounts of hexanes, heptanes and heavier components). Nitrogen is commonly found in natural gas. Other impurities in natural gas include helium, hydrogen sulfide, carbon dioxide, and water. Occasionally, cyclic and aromatic compounds occur in natural gas.
The composition of natural gas streams vary over a wide range as indicated by the data presented in Table 1. Nitrogen content in natural gas varies from a few percent to over 50%. The nitrogen may have formed naturally or have been introduced by injection into oil wells to stimulate production.
Gas pipeline transmission systems limit the allowable maximum content of non-combustible gases, nitrogen and carbon dioxide in natural gas because non-combustible gases lower heating value. The maximum specification for non-combustibles ranges typically between about 2 mol % and about 8 mol %. The Gas Research Institute estimates that there are about 55 trillion cubic feet of natural gas reserves in the continental United States that are "sub-quality". The Gas Research Institute defines natural gas containing more than 4 mole % nitrogen as sub-quality. These gas reserves must be processed to reject nitrogen and other non-combustibles to be acceptable to most pipelines. In addition, production of oil that is associated with high nitrogen gas is impeded because government regulations bar producers from burning the nitrogen-rich gas or venting the gas into the atmosphere.
Currently, the predominant technology for rejecting nitrogen from natural gas is cryogenic. The natural gas feed is liquefied by flashing the gas across Joule-Thompson valves or expanders to achieve very low temperatures (as low as -300.degree. F.). Then, nitrogen is fractionated overhead out of the liquefied natural gas in a distillation tower forming a bottoms stream of methane with low nitrogen content. Cryogenic processes are intensively heat exchange integrated to reduce power consumption, e.g., incoming feed is cooled by heat exchange against outgoing cold product streams, thus conserving most of the energy that went into refrigerating the product.
Nitrogen rejection adds significantly to the cost of producing natural gas. Currently, most high nitrogen reserves are not produced because the margin between the cost of bringing the gas to the surface at the wellhead and the market price of gas at the pipeline is not sufficient to pay for rejection. Cryogenic processes have been thoroughly optimized and there is little prospect that the cost of cryogenic separations can be significantly reduced. Cryogenic processes are inherently expensive because of the very low temperatures encountered. Feeds must be purified to very low concentrations of water, carbon dioxide and mercury because minute amounts of these impurities have deleterious effects at cryogenic temperatures. Exotic, costly materials are required to withstand cryogenic temperatures. The intensive heat exchange integration required to reduce energy consumption makes cryogenic units difficult to control and operate and impossible to adapt to changing feeds and product slates, which are often required in energy markets. Clearly, there is opportunity for new cost effective and flexible rejection technology which will allow production of our vast high nitrogen natural gas resources.